The present invention relates to rodent repelling devices and, more particularly, to an improved circuit for generating a substantially constant electromagnetic field for repelling rodents.
Various electrical devices have been proposed to repel rodents such as rats and mice from buildings and other structures. One such device is disclosed in U.S. Pat. No. 4,414,653 in which sound waves are generated at a frequency which is irritating to rats and mice. Another form of such device is shown in U.S. Pat. No. 4,802,057 in which a magnetic field is generated with a pulse frequency of approximately 60 cycles per minute, which frequency is known to repel rodents such as rats and mice. The apparatus illustrated in the '057 patent is relatively complicated and requires a rectifier and voltage regulator for producing low voltage DC electrical power from AC electrical power connected to the device. A low voltage circuit generates a pulsed frequency gating signal which gating signal is then applied to an amplifier and from the amplifier to an optical isolator circuit which separates the low voltage circuit from the high voltage control circuit. The high voltage control circuit includes a switch connected in series between a magnetic coil and the AC power source. The switch is controlled by the gating signal passing through the optical isolator. However, the switch also requires more power than is available from the optical isolator and in that regard AC power is coupled through a current limiting resistor to a gate input of the switch. The gating signal from the optical isolator, is summed with AC power and utilized as a gating signal to the switch. During the time that the pulsed gating signal from the optical isolator is coupled to the switch, the AC power is used to gate the switch into and out of conduction. One of the detriments with this system is that the switch is operated at the AC power frequency, i.e., 60 cycles per second (60 Hz), even though the magnetic field is desirably operated at 60 cycles per minute or at a frequency of 1 Hz. Furthermore, due to the phase shift between the voltage and current caused by the inductance of the coil, the circuit produces a reduced power output since the AC gating signals are applied in synchronism with the applied AC voltage across the coil and such voltage is phase shifted from the actual current through the coil. It is believed that the gating of the switch using the 60 Hz AC power signal out of phase with the current through the coil is the cause of a power variation in the electromagnetic field generated by the device illustrated in the '057 patent.